1. Field of Invention
This invention relates generally to the field of solution concentration, and more particularly concerns reverse osmosis systems, electrodialysis systems, brine concentrators, evaporators, desalinators and other systems that use preferential precipitation to control scaling and fouling.
2. Description of the Prior Art
It is known that relatively pure solvent can be removed from a solution by reverse osmosis (R.O.) wherein solvent is extracted by pressurizing a solution against a semi-permeable membrane that allows solvent to pass while obstructing the passage of solutes. For example, fresh water can be extracted from a brine solution by pressurizing the solution against an aromatic polyamide or cellulose acetate membrane that allows water to pass, but not salt. In this manner, solvents can be extracted from solutions, and the remaining solutes can be concentrated.
Evaporators and distillation systems are also used to purify solvents or concentrate solutes, though in a substantially different manner. In these systems, a solution is usually heated on a heat transfer surface and solvent is driven off by evaporation. If the object is to capture purified solvent, then the solvent vapors are condensed and retained. If the object is to capture or concentrate dissolved salts or suspended particles, then retaining condensed solvent vapors may not be essential.
Both reverse osmosis and evaporatory systems can be adversely affected by precipitating salts that can deposit on the working surface(s), fouling the system and forming scale. The resultant layer of precipitate reduces efficiency and can permanently damage the equipment by clogging or rupturing the membrane or by pitting or forming an insulating layer on the heat transfer surfaces(s).
The likelihood of scale forming depends on the nature of the solution being treated and the operating parameters of the system. A feed solution often contains a variety of very soluble and slightly soluble salts, with the slightly soluble salts being first to reach the saturation point where they precipitate and form scale. Salts in this category include calcium fluoride, calcium sulfate, calcium phosphate, silica, hydrated iron oxides, and other hydrated metal oxides. Other salts in this category are the fluoride, sulfate, and phosphate salts of other alkaline earth metals such as barium and strontium.
To alleviate the problems associated with scaling and fouling in solution concentration systems, a number of remedies have been proposed. These include chemical and physical pretreatment of feed solutions with chelating agents, solubility promoters and filters, and operating on large volumes of solution at very low rates of extraction.
Unfortunately, operating on large volumes of solution at low extraction rates is not always feasible or desirable and is often expensive. Physical and chemical pretreatment of feed solutions is also expensive because of the energy, capital, chemicals, and labor consumed. One alternative, preferential precipitation, promises to be an adequate, cost effective means of controlling scaling and fouling.
In preferential precipitation, scaling and fouling are avoided by allowing solute to preferentially deposit on a slurry of seed crystals suspended in the feed solution. When adequate seed crystal surface area is available, precipitating salts are deposited on the seed and carried away from the working surfaces of the system. This practice is taught in U.S. Pat. No. 4,207,183 issued to Herrigel, which patent is incorporated herein by reference. As taught in Herrigel, the surface area of seed crystal required to prevent scaling is preferably 5.0.times.10.sup.6 cm.sup.2 per gram of precipitating solute per minute (cm.sup.2 /g/min). Nucleation crystals generally ranging from about 1 to 100 microns in length, and preferably having an average length of about 10 microns, are used.
Applicant has discovered that under certain circumstances, scaling can sometimes occur despite the presence of what would normally be thought an adequate amount of seed crystal. Specifically, scaling may occur in systems while concentrating solutions at concentration factors at or below about 2, especially in those which operatre between 1 and 2, when only the expected minimum or preferable amount of seed crystal necessary to prevent scaling is initially present.
The term "concentration factor" (C.F.) connotes the degree of change effected by a solution concentration system. It is a comparison between feed solution and effluent, and can be defined mathematically in a number of ways. For example, it can be defined as the concentration of system reject divided by the concentration of system feed: EQU C.F.=[conc. sys. reject]/[conc. sys. feed]
This designation is useful when the feed solution is not saturated, or when the concentration factor is measured in terms of one of the highly soluble salts or ions in a solution saturated in less soluble salts. Using concentration to measure C.F. for saturated salts would result in a C.F. of 1, since the concentration of both feed and reject streams would be the same, with precipitate accounting for the solvent removed.
C.F. is often expressed in volumn terms as the volume of feed solution divided by the volume of reject solution: EQU C.F.=[Volume Feed]/[Volume Reject]
Expressed in volume terms, C.F. reflects the degree of change effected even in saturated solutions, and as used in this application, concentration factor is usually expressed in volume terms or in terms of concentration of some very soluble salt or ion in solution. There are minor variations between these two methods of expression, but as used herein, the two methods are considered interchangeable. A system concentrating at a concentration factor of 1 is generally not removing solvent and the concentration of system reject is identical to the system feed. A system concentrating at a C.F. of 2 produces a solution roughly twice as concentrated as its feed solution.
The unexpected scaling noted over low concentration factors can be attributed to conditions heretofore unappreciated by those skilled in the art.
Applicant recognized that adding excess seed crystal might eliminate the scaling observed. Since adding a large excess of seed crystal can result in a prematurely thick and unworkable slurry and since the heavier the slurry the more expensive it is to circulate, it is desirable to add only the minimum amount necessary to prevent scaling.
Likewise, applicant considered reducing crystal size, an approach equivalent to increasing the effective amount of available seed crystal. This technique might be used at low concentration factors to solve the aforementioned problems, but it has been found that crystals reduced in diameter to less than about 3 microns form a fouling layer by settling in the laminar flow of the feed stream. Here, they are held against the surface of the transport conduit by a Bernoulli effect, and scale growth can occur.
Other remedies are economically unattractive. For example, in brine concentrators practicing preferential precipitation, one could increase the size of the seed crystal reservoir while maintaining a relatively slow feed solution flow rate. In this manner, less feed solution would be concentrated with each pass across the heat transfer surface, and the available seed crystal surface area per unit volume of feed solution would be effectively increased. Unfortunately, the capital and operating costs associated with increasing the size of the reservoir and circulating less feed solution per pass outweigh the advantages obtained. With the increase in size, the circulators must work against a larger hydrostatic head per unit of extract so that product cost is increased.
Hence, a method of insuring the presence of adquate seed crystal in preferential precipiation system operating over low C.F.'s, while avoiding the early formation of a sludge-like or unworkable slurry, is desirable.